When radio communication is performed, the radio frequency of a transmitting end and the radio frequency of a receiving end are basically made to be the same. However, actually, a gap between them of about several ppm to several tens ppm occurs owing to the accuracy of their reference clocks. To estimate the frequency gap and correct it is called frequency offset compensation (Automatic Frequency Compensation; hereinafter abbreviated as AFC).
While analog communication was the mainstream, as the AFC, there was used a method in which the receiving end swept the frequency of its clock source in an arbitrary range to select a point having a high reception level, or the like. However, in these days when radio digital communication is the mainstream, there is used a method in which the frequency offset quantity is estimated on the basis of a digital signal obtained by the A/D conversion of a received signal demodulated to a baseband frequency band to correct the frequency gap.
Although various methods are used and examined as the method of the estimation of the frequency offset quantity, there is generally known a method in which a phase difference between a former reception data and a latter reception data is obtained to remove the difference value owing to data modulation for obtaining the frequency offset quantity.
In this case, because the initial pulling into synchronism takes time in case of the use of known signals, which are limited in number, there is proposed a method for detecting the frequency offset quantity by using unknown signals (data signals) for pursuing transmission efficiency.
Hereinafter, a conventional reception apparatus will be described by reference to FIG. 1 to FIG. 3. FIG. 1 is a block diagram showing the schematic configuration of a conventional reception apparatus; FIG. 2 is a block diagram showing the schematic configuration of the AFC section of the conventional reception apparatus; and FIG. 3 is graphs showing an example of an I-Q plane for illustrating frequency offsets. Incidentally, here, the reception apparatus for the use of mobile communication in the code division multiple access (CDMA) system is examined.
In FIG. 1, an antenna 1 receives a radio signal, and a radio modulation demodulation section 2 converts the received signal from a high frequency signal to a baseband signal and outputs it to a reception procession section 3. The reception procession section 3 is composed of an A/D conversion section 4, a correlation section 5, an AFC section 6, a decoding section 7, and a error correction section 8. The A/D conversion section 4 performs the A/D conversion processing of an input received signal, and the correlation section 5 composed of, for example, a matched filer detects a demodulated signal.
The AFC section 6 detects a frequency offset quantity on the basis of the demodulated signal output from the correlation section 5, and outputs the detected frequency offset quantity to the decoding section 7 and a clock source 10. The details will be described later.
The decoding section 7 performs the phase compensation processing of the input demodulated signal on the basis of the frequency offset quantity being an output of the AFC section 6, and then performs the soft decision processing of the processed signal. The error correction section 8 performs the codec processing such as the de-interleave processing and the error correction processing of the decided signal, and outputs the processed signal to a baseband signal processing section 9. The baseband signal processing section 9 obtains the received data from the received signal after the reception processing thereof was performed by the reception processing section 3, and also obtains and transmission data to output them to a transmission processing section 11.
The clock source 10 keeps a reference clock frequency, and corrects the reference clock frequency on the basis of the frequency offset quantity that is the output of the AFC section 6, and further outputs the reference clock frequency to the radio modulation demodulation section 2, the A/D conversion section 3 and the baseband signal processing section 9. The transmission processing section 11 performs the transmission processing of a transmission baseband signal to output it to the radio modulation demodulation section 2.
Next, the configuration of the AFC section 6 and the frequency offset detection operation thereof will be described by reference to FIG. 2 and FIG. 3.
When the frequency offset is not detected by the use of known signals but is detected by the use of unknown signals (data signals), a received demodulated signal Dm is situated in any one of the first quadrant to the fourth quadrant but can not be specified. Then, if it is supposed that the noise level is sufficiently low, the demodulated signal is situated at one point in each quadrant as shown in FIG. 3A in the case where a frequency offset does not exist, but the position of the demodulated signal shifts as the passage of time as shown in FIG. 3B in the case where the frequency offset θf exists.
Because the offset quantity θf between the received symbol that delayed by one symbol and the present received symbol is always constant, the offset quantity θf can be obtained by the operation of the difference between the received symbol that delayed by one symbol and the present received symbol.
Accordingly, a delay unit 21 delays the input received demodulated signal Dm by one symbol, and a subtracter 22 subtracts the output of the delay unit 21 from the present symbol, and further a phase detector 23 converts the subtraction result ΔDm of the subtracter 22 to a phase angle to detect a phase shift θm.
However, the phase shift θm is not equivalent to the frequency offset θf and the phase shift θm also includes a phase offset θd owing to the data modulation (θm=θd+θf). Consequently, it is necessary to remove the phase offset θd.
Now, if the modulation system is supposed to be the quadrature phase shift keying (QPSK), the phase offset θd is 0°, 90°, 180° and 270°. Because these values become multiples of 360° by the multiplication by four, the frequency offset θf can be obtained by the removal of θd from θm by the following computation formula.                                           (                                          (                                  4                  ×                  θ                  ⁢                                                                          ⁢                  m                                )                            ⁢                                                          ⁢                              mod                ⁡                                  (                                      360                    ⁢                    °                                    )                                                      )                    /          4                =                              (                                          (                                  4                  ×                                      (                                                                  θ                        ⁢                                                                                                  ⁢                        d                                            +                                              θ                        ⁢                                                                                                  ⁢                        f                                                              )                                                  )                            ⁢                              mod                ⁡                                  (                                      360                    ⁢                    °                                    )                                                      )                    /          4                                        =                              (                                          (                                                      4                    ⁢                    θ                    ⁢                                                                                  ⁢                    d                                    +                                      4                    ⁢                    θ                    ⁢                                                                                  ⁢                    f                                                  )                            ⁢                              mod                ⁡                                  (                                      360                    ⁢                    °                                    )                                                      )                    /          4                                        =                  4          ⁢          θ          ⁢                                          ⁢                      f            /            4                                                  =                  θ          ⁢                                          ⁢          f                    
Accordingly, the frequency offset θf is obtained by the following: the phase difference θm is multiplied by four by a multiplier 24, and the remainder when the output of the multiplier 24 is divided by 360° is calculated by a modulo (mod) arithmetic unit 25, and 4 θf is multiplied by ¼ by a multiplier 26. Then, last, an averaging section 27 averages the frequency offset quantity θf in an arbitrary interval to perform the estimation and the correction of the frequency offset quantity.
As described above, because the conventional frequency offset detecting method uses not only the limited known signals but also data signals, the shortening of the initial pulling time of the AFC is possible.
However, the conventional frequency offset detecting method has a problem that the accuracy of estimation can be deteriorated because the method uses the received signal at a step before the performance of the error correction processing.
Because the bit error ratio (BER) after error correction supposed in the future cellular system using CDMA or the like is about 10-3, the BER of a signal before the error correction is equal to or more than 10-1 when it is reckoned back. When the frequency offset quantity is estimated by the use of a signal having such a BER, the deterioration of the estimation accuracy becomes large and the initial pulling can be difficult.